Nothing, actually, which would answer all three of your questions. The worst I've ever had is a couple of pieces of Adware, back before I understood more about computers, and I cleaned those up really nicely by just going into the add/remove programs and killing the programs I didn't install. Through a combination of luck and security paranoia (and changing over my laptop to Linux), I've managed to avoid any actual viruses..
For those of you who may be less safe, let me make a few recommendations:
First and foremost, don't use Internet Explorer unless you absolutely have to (there are a few poorly written programs and websites that still require IE, but they're going the way of 624 KB RAM computers. It's made some improvements, but it still has some gaping security vulnerabilities, especially compared to other browsers. I use and recommend firefox, but more accurately, I recommend =not= IE.
Secondly, be careful about your websites. Sites that let you inject your own scripts (yes, MySpace, I'm talking about you) are really cool and awesome. You can force everyone who comes to your page to listen to that one song you really love. But someone much more sinister can force it to run pieces of code that can put adverse things on your computer. Using MySpace itself, for instance, can be fine, but be careful about whose pages you go to. Even if you know the person, they could have gone somewhere and gotten infected. Constant vigilance, as Professor Moody would say in Goblet of Fire .
Thirdly, if you do use Firefox, get a couple of lovely little addons called NoScript and AdBlock Plus. AdBlock is relatively self explanatory and less important, but it's really useful for browsing the Internet quickly. NoScript, on the other hand, will, by default, turn off things like JavaScript on any page that you visit. Which means right after you install it, most web pages look really weird, but whenever you visit a page that has scripts running, it'll show you a list of the sources of all scripts on that page, and you can choose which ones you want to allow. So, for instance, you can allow scripts from thefinalfantasy.net, but block that one from ilogyourkeys.com if it had somehow managed to worm its way into an advertisement.
Fourthly, if you do get infected, there's a lot of pretty decent software at helping remove as much of it as you can. I've always used Spybot: Search and Destroy, although i'm sure there's better stuff now. If you notice something, go after the problem right away: sitting around will only make it worse.
Or just switch to Linux ^_^.
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